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  3. A new kind of radar has been developed for space-age travelers. A working laboratory model of a new system of radar that makes u

A new kind of radar has been developed for space-age travelers. A working laboratory model of a new system of radar that makes u

admin2015-12-31  52
问题     A new kind of radar has been developed for space-age travelers. A working laboratory model of a new system of radar that makes use of a beam of light is said to be ten thousand times more accurate than the best comparable system of radar that uses microwaves.
    The model has shown that this radar system (known as laser-doppler radar) can measure with absolute precision speeds varying from spaceship orbital injection velocities (速度) of five miles per second down to virtual stops—speeds of less than one ten thousandth of an inch per second. According to the scientists who are developing this system, such fine measures of velocity are of prime importance in space missions. In a rendezvous (会合, 会合点) between two spaceships, or in a landing approach by a vehicle onto an orbiting space station, a bump could rip open a ship’s skin, or a nudge (轻触) could knock the station out of its orbit.
    The light-beam radar, which operates at a frequency of trillions of cycles per second, could easily detect and measure the movement of a vehicle edging up to a satellite space station even at a small fraction of an inch per second. A control system using so precise a signal as this would allow a huge vehicle to dock at a space station as lightly as a feather.
This laser-doppler radar will be especially valuable in space because________.
选项 A、the speeds of space vehicles will vary greatly
B、accurate measurements are essential in space
C、inaccuracies in measurement could cause accidents in space
D、all of the above
答案D
解析 文中第二段第二句、第三句说：“据正在开发这个系统的科学家说，这样精确的速度测量对于太空飞行来说是头等重要的事情。这种精确的速度测量的重要性在于，当处在两艘宇宙飞船的会合点上，或当一个飞行器在一个轨道太空站进行着陆时，一次轻微碰触就有可能把飞船的外壳撕开一个口子或把太空站撞出其运行轨道。”由此我们可以得出选项B和C都符合题意；又根据文中第二段第一句可知选项A符合题意。而选项D说A、B、C三个选项都对，故本题正确答案为D。
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